The visual impression of an actual material or object in an environment under defined illumination and viewing conditions is referred to in the relevant specialist circles as “appearance”. Due to this general usage, only this specialist term is used in the following.
Appearance is known to be the result of a complex interaction of different factors:                geometrical factors which define the scene, the object and the illumination and viewing conditions;        optical properties which describe the interaction between light and the material of the viewed object;        physiological factors which influence the perception (response) of the human visual system.        
Measuring “appearance” mainly relates to the second point, i.e. determining optical properties of the measurement object and their influence on the distribution of the reflected and transmitted light. Appearance measurement data are used to calculate appearance properties which correlate with the visual impression, such as for example colour. This is relevant for industrial applications such as for example product specification and quality control. Appearance measurement data also form the basis for simulating and predicting appearance in combination with physical models which replicate the interaction between light and the material of the measurement object. This is used in colour matching and in rendering.
Report No. 175 by the “Commission Internationale de l'Eclairage” (CIE), a 2006 report entitled “A Framework for the Measurement of Visual Appearance”, provides an overview of these topics. The report groups optical properties and associated appearance properties into four main categories: colour, gloss, texture and translucency.
The book “The Measurement of Appearance” by R. S. Hunter and R. W. Harold, 2nd edition, 1987, John Wiley & Sons, describes established measuring methods and standards for capturing specific appearance properties. In accordance with CIE 2006, for example, a total appearance capture (TAC) is when a complete set of appearance-relevant parameters is measured.
The optical properties and associated appearance properties can be differentiated into the spectrally dependent, the locally dependent and the angle-dependent.
Spectrally dependent properties relate to colour perception.
Texture is local variation in the surface of the viewed object, wherein two components are differentiated: visual texture characterises the inconsistency or colour heterogeneity of the surface; surface texture characterises the three-dimensional topography of the surface on a scale which the human eye can resolve.
Translucency also includes a local dependency. It describes the lateral diffusion of light by scattering and multiple reflection in the medium.
Colour and gloss are mainly determined by the angle-dependent reflection and transmission properties and represent locally integral properties. Gloss is dominant in an angular range near the specular reflection. The angle-dependent light distribution is influenced by the surface structure (for example, surface roughness) which is not visible (to the human eye). Colour is the important property further away from the specular direction into the “diffuse” reflection range.
The angle-dependent and location-dependent properties are influenced by the viewing distance. The surface texture is visible if the lateral size (scale) and the contrast of the structures are greater than the resolution capacity of the human eye. As the viewing distance increases, the texture gradually disappears and converges into the corresponding angle-dependent reflection and transmission variables.
The visual characterisation of appearance for materials with multiple different appearance properties is difficult and is typically only possible by visually comparing sample pairs with very similar optical properties. Appearance properties are linked to and dependent on each other. These reciprocal dependencies have to be taken into account in suitable appearance description models. For many industrial applications, colour is the appearance property which is most important and of primary interest. The colour impression is dependent on local and angle-dependent properties. Visual colour perception is influenced by adjacent colours and illumination conditions. Visible texture on the surface of the viewed object reduces the sensitivity for recognising colour differences. A high level of brightness and significant angle-dependent variation in brightness influence the perceived brightness and impair the recognition of colour differences. Perceived gloss is a multi-dimensional angle-dependent parameter and is influenced by the local distribution of the visible gloss over a textured surface.
The improved characterisation of materials with complex appearance properties requires new types of combined appearance measurement devices which enable all appearance properties to be systematically and reproducibly assessed.
One particularly important area of application for appearance measuring is the characterisation of special effect pigments such as are for example used in automobile paints.
One of the main properties of such effect pigments is their goniochromatic behaviour in the distant field, which requires multiple measurement geometries with different illumination and viewing angles in order to capture it. Various industrial standards each propose suitable sets of measurement geometries. ASTM E2194-03 defines a set of at least three measurement geometries for metallic effect pigments. ASTM E2539 defines additional measurement geometries for characterising interference pigments: a total of eight reference measurement geometries with two illumination angles 45° and 15° for capturing characteristic interference-related colour changes in the effect pigments.
Effect pigments are also known to produce visible textures on the surface of the viewed object. The visual texture is different under specular illumination, such as for example direct sunlight, and diffuse illumination, such as for example overcast skies. Specular illumination produces a pattern of very bright visible point light sources which are caused by direct reflections on the specular surfaces of the pigment flakes on the uppermost layer of paint. These point light sources are usually referred to as sparkle, glitter or micro-brilliance. Diffuse illumination produces a local variation in brightness with much deeper contrast, which is usually referred to as graininess, diffuse graininess, image grain or granularity. The publications “Observation and Measurement of the Appearance of Metallic Materials. Part I: Macro Appearance” in Color Research and Application, Volume 21, 292-304, 1996 by C. S. McCamy and “Observation and Measurement of the Appearance of Metallic Materials. Part II: Micro Appearance” in Color Research and Application, Volume 23, 362-373, 1998 by C. S. McCamy describe methods based on image measurement for characterising visual texture properties of materials with effect pigments. The publication “Observation of Visual Texture of Metallic and Pearlescent Materials” in Color Research and Application, Volume 32, 256-266, 2007 by E. Kirchner et al. provides a more up-to-date overview of these topics.
A range of hand-held measurement devices exist for measuring individual specific appearance properties such as colour and gloss, see the device portfolios of various relevant companies such as for example X-Rite, Datacolor, BYK Gardner, Konika and Minolta.
Examples include colour measurement devices with a circular 45°/0° measurement geometry or a d/8 measurement geometry with diffuse illumination from an integrator sphere, or hand-held measurement devices for specular gloss at one to three illumination angles (the device TRI-gloss by BYK Gardner).
The company Rhopoint manufactures a device Rhopoint IQ which, in addition to a three-angle measurement geometry for specular gloss in one channel, includes an additional detector field which serves to characterise other gloss properties related to the goniophotometric intensity distribution. This measuring design is not suitable for capturing colour and texture.
Measurement devices which are embodied to measure both colour and specular gloss are already known. Examples include the device Spectro-Guide by BYK Gardner and the device 45G by Datacolor. These devices each use two different measurement systems, for colour on the one hand (either a 45°/0° measurement geometry or a diffuse measurement geometry) and specular gloss on the other (a 60° measurement geometry with no colour information). These devices cannot produce consistent appearance measurement datasets, i.e. measurement datasets for different appearance properties, captured using the same measurement geometry. The different measurement values are not interrelated. These devices also cannot capture textures.
Multi-angle colour measurement devices for characterising effect pigments are also already known. These devices enable the spectral reflection factor to be measured using multiple measurement geometries, such as are for example defined in the ASTM standards. One representative example of such measurement devices is the device MA98 by X-Rite with two specular white illumination channels (15° and 45°) and ten specular measurement channels (pick-up channels) arranged partly in a plane and partly outside said plane.
The device BYKmac by BYK Gardner combines spectral multi-angle colour measurement with monochromatic texture measurement. While the two functionalities are combined in the same measurement device, they are realised by means of completely separate measurement systems for colour and texture. The texture measuring part does not include colour information. Colour information and texture information are obtained at different viewing angles and do not form a consistent dataset of the viewed object for a specific observation geometry. Spectral measurement value capture is implemented using an individual spectral light source and multiple integrally measuring detector systems. The texture is captured using white illumination and a monochromatic camera arranged at 0°. Simultaneous data capture is not possible using this device. Measuring requires a sequential process of colour capture and texture capture. The design comprising separate measurement systems and a sequential measuring process incurs time constraints and restricts the number of measurement geometries which can be realised. The device comprising the camera arranged at 0° is also not suitable for measuring gloss.
For laboratory applications, more general measuring instruments are already known. They include distant-field measurement systems which capture the bidirectional reflectance distribution function (BRDF, see for example F. E. Nicodemus et al., “Geometrical Considerations and Nomenclature for Reflectance”, National Bureau of Standards NIST report, 1977) in the half-space above the surface of the measurement object. While these known measuring instruments can capture spectral reflection properties for colour and for gloss using different measurement geometries, they are however relatively large due to their systems and therefore not suitable for realising a compact hand-held measurement device, in particular one with an additional texture capturing functionality. Examples of such measuring instruments include the goniospectrometric colour measurement device GSMS-3B by Murakami and the camera-based BRDF measurement devices Parousiameter by RadiantZemax and EZContrast by Eldim or the system described in the dissertation “Measuring and Modeling Anisotropic Reflectance”, Proceedings of the 19th annual conference on computer graphics and interactive techniques (SIGGRAPH '92), by G. Ward.
One way of capturing texture using a camera-based detector and different illumination geometries is described in the publication “Reflectance and Texture of Real-World Surfaces” in ACM Transactions on Graphics, Volume 18, No. 1, January 1999, 1-34 by K. Dana et al., wherein corresponding capturing systems are realised by means of moving robot arms for varying the measurement geometry. Such arrays are of course not suitable for hand-held measurement devices.